1. Field of the Invention
This invention relates to a monitoring system for a tube mill and more particularly to a monitoring system which ensures that the tubes formed in the tube mill will not have any weld defects therein.
2. Description of the Related Art
Tube mills have been utilized for many years for producing welded steel tubing. Welded steel tubing is fabricated from flat rolled steel strip received in coils from the steel mill.
The tube mill specifies thickness, width, chemical analysis and mechanical properties to ensure the production of tubing which meets mill quality standards and those of its customers.
Wide steel coils are slit to very accurate width, which is determined by as-welded tube size. Steel for making welded tubing is available either cold rolled or hot rolled.
Normally, welded steel tubing is formed, welded, sized and cut on one machine. The flat steel is normally passed through consecutive paired contoured rolls which gradually form the cold steel into a tubular shape. It then enters the welding unit where the butted edges are joined by one of several methods. Accurate and proper contact with the steel's edges is important in this operation.
Electric resistance welding (ERW) is the normal procedure for manufacturing carbon steel tubing, as well as many alloy steels. In ERW, the heat for welding the butted edges together is generated by the resistance of the steel to the flow of an electric current. The heat is confined to a narrow band along the edges with the highest temperature at the extreme edges.
In one form of ERW, a high frequency current is conducted to the strip edges by an induction coil. In the embodiment of the present invention, the induction coil is circumferential, and makes no direct contact with the tubing. The high frequency current is passed through the coil and sufficient energy is induced in the tubing to bring the butted edges to welding temperature. A set of rolls squeeze the butted edges together while they are at welding temperature to complete the weld. No extraneous metal is added during such a welding operation.
The electric resistance welding procedure extrudes a small amount of metal (weld flash) on both the inside and outside of the tubing. It does not affect the properties of the tubing, because it is solid metal. This weld flash is removed from the outside diameter with a cutting or planing tool in order to keep the outside diameter uniform. The tool is positioned on the welding machine immediately after the welding unit.
In order to create uniform weld along the tubing, the welding temperature should be maintained within a specific range for the specific type of material. In addition, appropriate pressure on the steel strips edges is required to maintain the accurate and proper contact during the welding process for uniform welds.
After being welded, the tubing enters the sizing section of the process, wherein a set of opposing pairs of rolls will size and form the tube into the desired configuration. The tubing is then advanced to a cut off mechanism, wherein the desired length of tube is cut off "on the fly".
Heretofore, when defects were discovered in the finished product, those defective sections were cut from the tube. However, in the past, even with the methods of testing which have been employed, defective tubing could be shipped from the tube mill.
In the prior art, random portions of tubing were selected for destructive testing procedures. For example, in a flattening test, a force is applied to the tubing surface at right angles to its axis. The weld is positioned at 90.degree. or 0.degree. to the applied force, in order to check the weld quality. A crushing test is applied to short lengths of tubing, and results in a fold which stretches the outside fibers at the surface. Flare and flange tests utilize short lengths of tubing which are forced over a cone having an angle of about 60.degree.. A flange is produced when the tubing is flared over the cone and then placed on a flat surface under increased load. This is the most common test, because it stretches the tubing, including the weld, to the extent desired and will eventually cause the metal to fail. Preferably, the metal of the tubing will fail before the metal in the weld portion of the flare.
In recent years, steel tubing has been utilized in the production of door beams for vehicles. For example, see U.S. Pat. No. 4,636,608 which discloses an apparatus for cutting the ends of the tubes or pipes so that they may be positioned in the door of a vehicle. Since the door beams are being utilized as a safety feature in the vehicle, it is imperative that the beams employed therein be completely free from any weld defects which could affect their performance in the event of a collision. Heretofore, it was not possible to be able to ensure that the steel tubes being supplied for the door beams were completely free of defects which could affect their performance in a crash.